Sunho Oh

In vivo study on hydroxyapatite scaffolds with trabecular architecture for bone repair.

The objective of this research was to investigate the bone formation and angio-conductive potential of hydroxyapatite (HA) scaffolds closely matched to trabecular bone in a canine segmental defect after 3 and 12 weeks post implantation. Histomorphometric comparisons were made between naturally forming trabecular bone (control) and defects implanted with scaffolds fabricated with micro-size (M-HA) and nano-size HA (N-HA) ceramic surfaces. Scaffold architecture was similar to trabecular bone formed in control defects at 3 weeks. No significant differences were identified between the two HA scaffolds; however, significant bone in-growth was observed by 12 weeks with 43.9 +/- 4.1% and 50.4 +/- 8.8% of the cross-sectional area filled with mineralized bone in M-HA and N-HA scaffolds, respectively. Partially organized, lamellar collagen fibrils were identified by birefringence under cross-polarized light at both 3 and 12 weeks post implantation. Substantial blood vessel infiltration was identified in the scaffolds and compared with the distribution and diameter of vessels in the surrounding cortical bone. Vessels were less numerous but significantly larger than native cortical Haversian and Volkmann canals reflecting the scaffold architecture where open spaces allowed interconnected channels of bone to form. This study demonstrated the potential of trabecular bone modeled, highly porous and interconnected, HA scaffolds for regenerative orthopedics.

The integration of chitosan-coated titanium in bone: an in vivo study in rabbits.

Procedure:Much research is directed at surface modifications to enhance osseointegration of implants. A new potential coating is the biopolymer, chitosan, the deacetylated derivative of the natural polysaccharide, chitin. Chitosan is biocompatible, degradable, nontoxic, and exhibits osteogenic properties. The aim of this research was to investigate the hypothesis that chitosan-coated titanium supports bone formation and osseointegration. Materials and Methods:Chitosan (1wt% of 92.3% deacetylated chitosan in 1% acetic acid) was solution cast and bonded to rough ground titanium pins (2-mm diameter × 4-mm long) via silane reactions. Calcium phosphate sputter-coated titanium and uncoated titanium pins were used as controls. Two chitosan-coated pins, and 1 each of calcium phosphate coated and uncoated pins were implanted unilaterally in the tibia of 16 adult male New Zealand white rabbits. At 2, 4, 8, and 12 weeks, undecalcified sections were histologically evaluated for healing and bone formation. Results:Histological evaluations of tissues in contact with the chitosan-coated pins indicated minimal inflammatory response and a typical healing sequence of fibrous, woven bone formation, followed by development of lamellar bone. These observations were similar to those for tissues interfacing the control calcium phosphate-coated and uncoated titanium implants. Quantitative comparisons of the bone-implant interface were not possible since 31% of the implants migrated into the tibial marrow space after implantation due to insufficient cortical bone thickness to hold pins in place during healing. Conclusion:These data support the hypothesis that chitosan-coatings are able to develop a close bony apposition or the osseointegration of dental/craniofacial and orthopedic implants.

A Cellular Perspective to Bioceramic Scaffolds for Bone Tissue Engineering: the State of the Art

A vast number of manufacturing techniques have been employed in the last five years to manufacture three dimensional (3D) calcium phosphate (CaP) scaffolds, with the intention to replicate the architecture of native bone as well as to repair and restore bone function. Design features such as architectural control and sintering temperature and their impact on scaffold performance is presented in this review. In vitro cell responses to bioceramic scaffolds and their in vivo performances have been enhanced. Current frontiers of active research on HA scaffolds have included the relationship between fluid flow and mechanotransduction as well as cell signaling pathways that induce endothelial cell recruitment and angiogenesis. Additionally, current research has focused on a better understanding of cell signaling and its environmental cues. The availability of non-invasive and non-destructive quantitative imaging modalities has also become critical in aiding the characterization of scaffolds and predicting scaffold performance. It is thus anticipated that further knowledge gained from this research will allow the overall advancement of scaffolds that can be clinically used to restore large bone defects.

Hydroxyapatite/polylactide biphasic combination scaffold loaded with dexamethasone for bone regeneration.

This study presents a novel design of a ceramic/polymer biphasic combination scaffold that mimics natural bone structures and is used as a bone graft substitute. To mimic the natural bone structures, the outside cortical-like shells were composed of porous hydroxyapatite (HA) with a hollow interior using a polymeric template-coating technique; the inner trabecular-like core consisted of porous poly(D,L-lactic acid) (PLA) that was loaded with dexamethasone (DEX) and was directly produced using a particle leaching/gas forming technique to create the inner diameter of the HA scaffold. It was observed that the HA and PLA parts of the fabricated HA/PLA biphasic scaffold contained open and interconnected pore structures, and the boundary between both parts was tightly connected without any gaps. It was found that the structure of the combination scaffold was analogous to that of natural bone based on micro-computed tomography analysis. Additionally, the dense, uniform apatite layer was formed on the surface of the HA/PLA biphasic scaffold through a biomimetic process, and DEX was successfully released from the PLA of the biphasic scaffold over a 1-month period. This release caused human embryonic palatal mesenchyme cells to proliferate, differentiate, produce ECM, and form tissue in vitro. Therefore, it was concluded that this functionally graded scaffold is similar to natural bone and represents a potential bone-substitute material.

Delivery of paclitaxel from cobalt-chromium alloy surfaces without polymeric carriers

Polymer-based carriers are commonly used to deliver drugs from stents. However, adverse responses to polymer coatings have raised serious concerns. This research is focused on delivering drugs from stents without using polymers or any carriers. Paclitaxel (PAT), an anti-restenotic drug, has strong adhesion towards a variety of material surfaces. In this study, we have utilized such natural adhesion property of PAT to attach these molecules directly to cobalt-chromium (Co-Cr) alloy, an ultra-thin stent strut material. Four different groups of drug coated specimens were prepared by directly adding PAT to Co-Cr alloy surfaces: Group-A (PAT coated, unheated, and ethanol cleaned); Group-B (PAT coated, heat treated, and ethanol cleaned); Group-C (PAT coated, unheated, and not ethanol cleaned); and Group-D (PAT coated, heat treated and not ethanol cleaned). In vitro drug release of these specimens was investigated using high performance liquid chromatography. Groups A and B showed sustained PAT release for up to 56 days. A simple ethanol cleaning procedure after PAT deposition can remove the loosely bound drug crystals from the alloy surfaces and thereby allowing the remaining strongly bound drug molecules to be released at a sustained rate. The heat treatment after PAT coating further improved the stability of PAT on Co-Cr alloy and allowed the drug to be delivered at a much slower rate, especially during the initial 7 days. The specimens which were not cleaned in ethanol, Groups C and D, showed burst release. PAT coated Co-Cr alloy specimens were thoroughly characterized using scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. These techniques were collectively useful in studying the morphology, distribution, and attachment of PAT molecules on Co-Cr alloy surfaces. Thus, this study suggests the potential for delivering paclitaxel from Co-Cr alloy surfaces without using any carriers.

Stability of antibacterial self-assembled monolayers on hydroxyapatite

Open fractures are common in battlefields, motor vehicle accidents, gunshot wounds, sports injuries, and high-energy falls. Such fractures are treated using hydroxyapatite (HA)-based bone graft substitutes. However, open fracture wounds are highly susceptible to bacterial infections. Hence, this study was focused on incorporating antibacterial properties to HA using silver (Ag) carrying self-assembled monolayers (SAMs). Also, the stability of Ag carrying SAMs on HA was investigated under sterilization and physiological conditions. Initially, the -COOH terminated phosphonic acid SAMs of two different chain lengths (11 carbon atoms - shorter chain and 16 carbon atoms - longer chain) were deposited on HA. Antibacterial SAMs (ASAMs) were prepared by chemically attaching Ag to shorter and longer chain SAMs coated HA. X-ray photoelectron spectroscopy, atomic force microscopy, and contact angle goniometry collectively confirmed the attachment of Ag onto SAMs coated HA. The bacterial adhesion study showed that the adherence of Staphylococcus aureus was significantly reduced on ASAMs coated HA when compared to control-HA. The stability studies showed that gas plasma, dry heat and autoclave degraded most of the ASAMs on HA. UV irradiation did not damage the shorter chain ASAMs as vigorously as other treatments, while it degraded the longer chain ASAMs completely. Ethylene oxide treatment did not degrade the longer chain ASAMs unlike all other treatments but it severely damaged the shorter chain ASAMs. Both shorter and longer chain ASAMs significantly desorbed from the HA surfaces under physiological conditions although longer chain ASAMs exhibited better stability than shorter chain ASAMs. This study demonstrated the potential for using ASAMs to provide antibacterial properties to HA and the need for developing techniques to improve stability of SAMs under sterilization and physiological conditions.

Paclitaxel delivery from cobalt-chromium alloy surfaces using self-assembled monolayers.

Polymer-based platforms in drug-eluting stents (DESs) can cause adverse reactions in patients. Hence, the development of a polymer-free drug delivery platform may reduce adverse reactions to DES. In this study, the use of a polymer-free platform, self-assembled monolayers (SAMs), is explored for delivering an antiproliferative drug [paclitaxel (PAT)] from a stent material [cobalt-chromium ((Co-Cr) alloy]. Initially, carboxylic acid terminated phosphonic acid SAMs were coated on Co-Cr alloy. Two different doses (25 and 100 μg/cm²) of PAT were coated on SAM coated Co-Cr surfaces using a microdrop deposition method. Also, control experiments were carried out to coat PAT directly on Co-Cr surfaces with no SAM modification. The PAT coated specimens were characterized using the Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and atomic force microscopy (AFM). FTIR spectra showed the successful deposition of PAT on SAM coated and control-Co-Cr surfaces. SEM images showed islands of high density PAT crystals on SAM coated surfaces, while low density PAT crystals were observed on control-Co-Cr alloy. AFM images showed molecular distribution of PAT on SAM coated as well as control-Co-Cr alloy surfaces. In vitro drug release studies showed that PAT was released from SAM coated Co-Cr surfaces in a biphasic manner (an initial burst release in first 7 days was followed by a slow release for up to 35 days), while the PAT was burst released from control-Co-Cr surfaces within 1-3 days. Thus, this study demonstrated the use of SAMs for delivering PAT from Co-Cr alloy surfaces for potential use in drug-eluting stents.

Migration of Co-cultured Endothelial Cells and Osteoblasts in Composite Hydroxyapatite/Polylactic Acid Scaffolds

Regeneration of bone in large segmental bone defects requires regeneration of both cortical bone and trabecular bone. A scaffold design consisting of a hydroxyapatite (HA) ring surrounding a polylactic acid (PLA) core simulates the structure of bone and provides an environment for indirect and direct co-culture conditions. In this experiment, human umbilical vein endothelial cells (EC) and normal human primary osteoblasts (OB) were co-cultured to evaluate cell migration and interactions within this biphasic composite scaffold. Both cell types were able to migrate between the different material phases of the scaffold. It was also observed that OB migration increased when they were co-cultured with ECs, whereas EC migration decreased in co-culture. The results show that co-culture of ECs and OBs in this composite biphasic scaffold allows for migration of cells throughout the scaffold and that pre-seeding a scaffold with ECs can increase OB infiltration into desired areas of the scaffold.

Evaluation of BMP-2 tethered polyelectrolyte coatings on hydroxyapatite scaffolds in vivo

The goal of this in vivo study was to evaluate the osteoinductive and angio-inductive properties of a porous hydroxyapatite (HAp) scaffold with immobilized recombinant bone morphogenetic protein-2 (rhBMP-2) on the surface. It was hypothesized in this study that the use of a rhBMP-2 incorporated polyelectrolyte coating on the HAp scaffold would allow for controlled exposure of rhBMP-2 into the tissue and would provide a sound platform for tissue growth. The scaffolds were characterized for porosity and interconnectivity using pycnometry, scanning electron microscopy and micro-ct. These scaffolds were then divided into the following four groups: (a) HAp scaffold (n-HAp group), (b) rhBMP-2 physically adsorbed on HAp scaffold (HAp-BMP-2 Group), (c) polyelectrolyte coating on HAp scaffold without rhBMP-2 (HAp-PEI Scaffold Group), and (d) polyelectrolyte coating tethered with rhBMP-2 on HAp scaffold (HAp-PEI-BMP-2 Scaffold Group). Using 18 skeletally matured New Zealand white rabbits, these scaffolds were evaluated in a nonload bearing femoral condyle plug model. The negative controls for this study have defects that were left untreated and the positive controls have defects that were filled with autologous bone graft harvested from epsilateral iliac crest. Bone induction, vessel growth, and scaffold-bone contact were analyzed after 8-week implantation using micro-CT and histomorphometry. It was concluded from this study that the use of scaffold with an attached rhBMP-2 increased the vascularization around the implant when compared with the uncoated n-HAp scaffold, a necessary step of bone regeneration. The open-pore HAp scaffold was also concluded to provide a platform for tissue growth, drug loading, and tissue interaction.

Calcium phosphate coating produced by a sputter deposition process

The properties of implant surfaces play critical roles in inducing a biological response. In the case of dental and orthopedic implants, deposition of calcium phosphate (CaP) coatings on these implant surfaces are often employed as means of enhancing implant osseointegration with the bone. Although most implants are coated using a plasma spraying process, sputtering is currently being accepted by some implant vendors as one of the means for depositing thin CaP coatings on dental and orthopedic implants. Acceptance of the sputtering technology and recent research are indications that the sputtering process is promising and has potential for eliminating some of the problems associated with the plasma-spraying process. This chapter discusses some of the various modes of sputtering, properties of thin CaP coatings, and the biological responses to these coatings in vitro and in vivo. The limitations and strengths of the sputtering process are also addressed.